1. Field of the Invention
The present invention relates to a process for vapor-phase epitaxial growth of a III-V compound semiconductor by the chloride CVD process. It also relates to a process for vapor-phase epitaxial growth of a semiconductor layer including a .delta.-doped layer in which an impurity dopes the semiconductor layer in a .delta.-function deposition profile.
2. Description of the Related Art
FIG. 8 illustrates a prior-art vapor-phase epitaxial reactor system for use in vapor-phase epitaxy on a substrate for a III-V compound semiconductor, such as gallium arsenide (GaAs), gallium phosphide (GaP), indium phosphide (InP) or indium arsenide (InAs), added with an impurity.
This vapor-phase epitaxial reactor system comprises a horizontal cylindrical reactor vessel 1 made of quartz and an electric furnace 2. The electric furnace 2 is designed to control the temperature distribution along the longitudinal axis of the reactor vessel 1. When the vapor-phase expitaxial reactor system is used for growing a gallium-arsenide crystal, a boat 4 for raw material is placed in the reactor vessel 1 at its upstream end (i.e., at the left-hand end). A source of gallium 3 is placed in the boat 4 and an epitaxial substrate of gallium arsenide 5 is placed near the downstream end of the reactor vessel 1.
The upstream end of the reactor vessel 1 communicates with a first gas-introduction conduit 6a and a parallel second gas-introduction conduit 6b. The gas-introduction conduits 6a and 6b feed gases to the substrate 5 upstream thereof, bypassing the boat 4. The upstream end of the reactor vessel 1 also communicates with a third gas-introduction conduit 6c feeding a gas to the boat 4.
The second gas-introduction conduit 6b has an intermediate bubbler 8b which communicates therewith and contains liquid arsenic trichloride (AsCl.sub.3). The third gas-introduction conduit 6c has an intermediate bubbler 8c which communicates therewith and contains liquid arsenic trichloride. The gas-introduction conduits 6b and 6c receive hydrogen gas from the outside and blow it into the liquid arsenic trichloride contained in the bubblers 8b and 8c so as to feed a mixture of arsenic trichloride gas and hydrogen gas into the reactor vessel 1. The bubblers 8b and 8c are placed in thermostatic ovens (not shown) so as to control the amount of the liquid arsenic trichloride evaporated. A waste gas discharge conduit 9 communicates with the downstream end of the reactor vessel 1.
The gas feed line including the first gas-introduction conduit 6a will hereinafter be referred to as a line A, the gas feed line including the bubbler 8b and the second gas-introduction conduit 6b as a line B, and the gas feed line including the bubbler 8c and the third gas-introduction conduit 6c as a line C.
In semiconductor devices having a gallium arsenide substrate, it has recently become the practice to provide the substrate with a .delta.-doped layer since an FET in a semiconductor device which includes a .delta.-doped layer in its vapor-phase epitaxial layer has a higher gm.
However, it is hard to grow a vapor-phase epitaxial layer of gallium arsenide crystal including the .delta.-doped layer by the chloride CVD process using the vapor-phase epitaxial reactor system of FIG. 8 because the epitaxial growth rate is as high as 10 .mu.m/hr and the epitaxy cannot be interrupted. FIG. 9 shows a vapor-phase epitaxial reactor system developed for overcoming this problem. This system includes two chambers for sourcing III group elements and during epitaxy the epitaxial substrate is transferred from one growth zone facing one chamber for sourcing a III group element to another growth zone facing the other chamber for sourcing another III group element so as to grow the .delta.-doped layer. The process using the FIG. 9 system requires two sourcing chambers and is therefore complex and costly.